Let's remove the debug level. The reporter stream is not a generic logger, and we have other ways (NODE_DEBUG) of adding debug output.

I will remove that. And I have a question so this CI https://github.com/nodejs/node/actions/runs/11983534043/job/33427085217?pr=55964 failed and it's for First commit message adheres to guidelines / lint-commit-message (pull_request) so should I change commit history??

test_runner: add level to diagnostics

So I am asking if I should change the commit history and force push.

Ah. Yes, please. You'll need to avoid merge commits too, as the tooling does not handle them well.

@pmarchini Ok so I am taking reference from this tests
https://github.com/nodejs/node/blob/main/test/fixtures/test-runner/output/coverage-width-80-color.mjs
https://github.com/nodejs/node/blob/main/test/fixtures/test-runner/output/coverage-width-80-color.snapshot

and based on that I wrote this test.
fixtures/test-runner/output/spec_reporter_diagnostic_levels.mjs

// Flags: --test-reporter=spec

import { test } from 'node:test';
import { TestsStream } from '../../../lib/internal/test_runner/tests_stream.js';

process.env.FORCE_COLOR = '3';

const testsStream = new TestsStream();

test('Diagnostic Levels Color Output', () => {
  testsStream.diagnostic(1, {}, 'Info-level message', 'info');
  testsStream.diagnostic(1, {}, 'Warning-level message', 'warn');
  testsStream.diagnostic(1, {}, 'Error-level message', 'error');
});

and add this in test-runner-output.mjs

{
  name: 'test-runner/output/spec_reporter_diagnostic_levels.mjs',
  transform: specTransform,
  tty: true,
}

now I am running tools/test.py test/parallel/test-runner-output.mjs --snapshot but it's not creating snap for added test.
How I can create snaps??

I got this when I tried using test with release/node

I forgot you were working in a Windows environment. Considering this, I would suggest picking a different approach. You could add a test to node/test/parallel/test-runner-coverage-thresholds.js that forces the colors via the environment variable in the spawn, and check that the error message 'coverage does not meet...' is displayed in red.
Note: you need to set the reporter to spec.

This is also because test-runner-output.mjs is intended for 'e2e' testing of the test runner's output under specific circumstances.

@pmarchini @cjihrig Thank you so much for your help. It wouldn't be possible without your help.
I ran test cases and here is output
I printed raw output in test case and here is output

and here is test case:

test(`test failing ${coverage.flag} with red color`, () => {
    const result = spawnSync(process.execPath, [
      '--test',
      '--experimental-test-coverage',
      `${coverage.flag}=99`,
      '--test-reporter', 'spec',
      fixture,
    ], {
      env: { ...process.env, FORCE_COLOR: '3' },
    });

    const stdout = result.stdout.toString();
    const redColorRegex = /\u001b\[31mℹ Error: \d{2}\.\d{2}% \w+ coverage does not meet threshold of 99%/;
    assert.match(stdout, redColorRegex, 'Expected red color code not found in diagnostic message');
    assert.strictEqual(result.status, 1);
    assert(!findCoverageFileForPid(result.pid));
  });

If it looks good to you I can commit it and you can review the whole PR.

Hi @pmarchini @cjihrig @jasnell @MoLow
Could we please land this PR if no change is required??